Network Working Group J. Strand, Ed.
Request for Comments: 4054 A. Chiu, Ed.
Category: Informational AT&T
May 2005
Impairments and Other Constraints on Optical Layer Routing
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
Optical networking poses a number challenges for Generalized Multi-
Protocol Label Switching (GMPLS). Fundamentally, optical technology
is an analog rather than digital technology whereby the optical layer
is lowest in the transport hierarchy and hence has an intimate
relationship with the physical geography of the network. This
contribution surveys some of the aspects of optical networks that
impact routing and identifies possible GMPLS responses for each: (1)
Constraints arising from the design of new software controllable
network elements, (2) Constraints in a single all-optical domain
without wavelength conversion, (3) Complications arising in more
complex networks incorporating both all-optical and opaque
architectures, and (4) Impacts of diversity constraints.
Table of Contents
1. Introduction ................................................. 22. Sub-IP Area Summary and Justification of Work ................ 33. Reconfigurable Network Elements .............................. 33.1. Technology Background .................................. 33.2. Implications for Routing ............................... 64. Wavelength Routed All-Optical Networks ....................... 64.1. Problem Formulation .................................... 74.2. Polarization Mode Dispersion (PMD) ..................... 84.3. Amplifier Spontaneous Emission ......................... 9
4.4. Approximating the Effects of Some Other
Impairments Constraints ................................ 104.5. Other Impairment Considerations ........................ 13Strand & Chiu Informational [Page 1]

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4.6. An Alternative Approach - Using Maximum
Distance as the Only Constraint ........................ 134.7. Other Considerations ................................... 154.8. Implications for Routing and Control Plane Design ...... 155. More Complex Networks ........................................ 176. Diversity .................................................... 196.1. Background on Diversity ................................ 196.2. Implications for Routing ............................... 237. Security Considerations ...................................... 238. Acknowledgements ............................................. 249. References ................................................... 259.1. Normative References ................................... 259.2. Informative References ................................. 2610. Contributing Authors ......................................... 261. Introduction
Generalized Multi-Protocol Label Switching (GMPLS) [Mannie04] aims to
extend MPLS to encompass a number of transport architectures,
including optical networks that incorporate a number of all-optical
and opto-electronic elements, such as optical cross-connects with
both optical and electrical fabrics, transponders, and optical add-
drop multiplexers. Optical networking poses a number of challenges
for GMPLS. Fundamentally, optical technology is an analog rather
than digital technology whereby the optical layer is lowest in the
transport hierarchy and hence has an intimate relationship with the
physical geography of the network.
GMPLS already has incorporated extensions to deal with some of the
unique aspects of the optical layer. This contribution surveys some
of the aspects of optical networks that impact routing and identifies
possible GMPLS responses for each. Routing constraints and/or
complications arising from the design of network elements, the
accumulation of signal impairments, and the need to guarantee the
physical diversity of some circuits are discussed.
Since the purpose of this document is to further the specification of
GMPLS, alternative approaches to controlling an optical network are
not discussed. For discussions of some broader issues, see
[Gerstel2000] and [Strand02].
The organization of the contribution is as follows:
- Section 2 is a section requested by the sub-IP Area management for
all new documents. It explains how this document fits into the
Area and into the IPO WG, and why it is appropriate for these
groups.
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- Section 3 describes constraints arising from the design of new
software controllable network elements.
- Section 4 addresses the constraints in a single all-optical domain
without wavelength conversion.
- Section 5 extends the discussion to more complex networks and
incorporates both all-optical and opaque architectures.
- Section 6 discusses the impacts of diversity constraints.
- Section 7 deals with security requirements.
- Section 8 contains acknowledgments.
- Section 9 contains references.
- Section 10 contains contributing authors' addresses.
2. Sub-IP Area Summary and Justification of Work
This document merges and extends two previous expired Internet-Drafts
that were made IPO working group documents to form a basis for a
design team at the Minneapolis IETF meeting, where it was also
requested that they be merged to create a requirements document for
the WG.
In the larger sub-IP Area structure, this merged document describes
specific characteristics of optical technology and the requirements
they place on routing and path selection. It is appropriate for the
IPO working group because the material is specific to optical
networks. It identifies and documents the characteristics of the
optical transport network that are important for selecting paths for
optical channels, which is a work area for the IPO WG. The material
covered is directly aimed at establishing a framework and
requirements for routing in an optical network.
3. Reconfigurable Network Elements3.1. Technology Background
Control plane architectural discussions (e.g., [Awduche99]) usually
assume that the only software reconfigurable network element is an
optical layer cross-connect (OLXC). There are however other software
reconfigurable elements on the horizon, specifically tunable lasers
and receivers and reconfigurable optical add-drop multiplexers
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(OADM). These elements are illustrated in the following simple
example, which is modeled on announced Optical Transport System (OTS)
products:
+ +
---+---+ |\ /| +---+---
---| A |----|D| X Y |D|----| A |---
---+---+ |W| +--------+ +--------+ |W| +---+---
: |D|-----| OADM |-----| OADM |-----|D| :
---+---+ |M| +--------+ +--------+ |M| +---+---
---| A |----| | | | | | | |----| A |---
---+---+ |/ | | | | \| +---+---
+ +---+ +---+ +---+ +---+ +
D | A | | A | | A | | A | E
+---+ +---+ +---+ +---+
| | | | | | | |
Figure 3-1: An OTS With OADMs - Functional Architecture
In Fig. 3-1, the part that is on the inner side of all boxes labeled
"A" defines an all-optical subnetwork. From a routing perspective
two aspects are critical:
- Adaptation: These are the functions done at the edges of the
subnetwork that transform the incoming optical channel into the
physical wavelength to be transported through the subnetwork.
- Connectivity: This defines which pairs of edge Adaptation
functions can be interconnected through the subnetwork.
In Fig. 3-1, D and E are DWDMs and X and Y are OADMs. The boxes
labeled "A" are adaptation functions. They map one or more input
optical channels assumed to be standard short reach signals into a
long reach (LR) wavelength or wavelength group that will pass
transparently to a distant adaptation function. Adaptation
functionality that affects routing includes:
- Multiplexing: Either electrical or optical TDM may be used to
combine the input channels into a single wavelength. This is done
to increase effective capacity: A typical DWDM might be able to
handle 100 2.5 Gb/sec signals (250 Gb/sec total) or 50 10 Gb/sec
(500 Gb/sec total); combining the 2.5 Gb/sec signals together thus
effectively doubles capacity. After multiplexing the combined
signal must be routed as a group to the distant adaptation
function.
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- Adaptation Grouping: In this technique, groups of k (e.g., 4)
wavelengths are managed as a group within the system and must be
added/dropped as a group. We will call such a group an
"adaptation grouping". Examples include so called "wave group"
and "waveband" [Passmore01]. Groupings on the same system may
differ in basics such as wavelength spacing, which constrain the
type of channels that can be accommodated.
- Laser Tunability: The lasers producing the LR wavelengths may have
a fixed frequency, may be tunable over a limited range, or may be
tunable over the entire range of wavelengths supported by the
DWDM. Tunability speeds may also vary.
Connectivity between adaptation functions may also be limited:
- As pointed out above, TDM multiplexing and/or adaptation grouping
by the adaptation function forces groups of input channels to be
delivered together to the same distant adaptation function.
- Only adaptation functions whose lasers/receivers are tunable to
compatible frequencies can be connected.
- The switching capability of the OADMs may also be constrained.
For example:
o There may be some wavelengths that can not be dropped at all.
o There may be a fixed relationship between the frequency dropped
and the physical port on the OADM to which it is dropped.
o OADM physical design may put an upper bound on the number of
adaptation groupings dropped at any single OADM.
For a fixed configuration of the OADMs and adaptation functions
connectivity will be fixed: Each input port will essentially be
hard-wired to some specific distant port. However this connectivity
can be changed by changing the configurations of the OADMs and
adaptation functions. For example, an additional adaptation grouping
might be dropped at an OADM or a tunable laser retuned. In each case
the port-to-port connectivity is changed.
These capabilities can be expected to be under software control.
Today the control would rest in the vendor-supplied Element
Management system (EMS), which in turn would be controlled by the
operator's OSes. However in principle the EMS could participate in
the GMPLS routing process.
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RFC 4054 Optical Layer Routing May 20053.2. Implications for Routing
An OTS of the sort discussed in Sec. 3.1 is essentially a
geographically distributed but blocking cross-connect system. The
specific port connectivity is dependent on the vendor design and also
on exactly what line cards have been deployed.
One way for GMPLS to deal with this architecture would be to view the
port connectivity as externally determined. In this case the links
known to GMPLS would be groups of identically routed wavebands. If
these were reconfigured by the external EMS the resulting
connectivity changes would need to be detected and advertised within
GMPLS. If the topology shown in Fig. 3-1 became a tree or a mesh
instead of the linear topology shown, the connectivity changes could
result in Shared Risk Link Group (SRLG - see Section 6.2) changes.
Alternatively, GMPLS could attempt to directly control this port
connectivity. The state information needed to do this is likely to
be voluminous and vendor specific.
4. Wavelength Routed All-Optical Networks
The optical networks deployed until recently may be called "opaque"
([Tkach98]): each link is optically isolated by transponders doing
O/E/O conversions. They provide regeneration with retiming and
reshaping, also called 3R, which eliminates transparency to bit rates
and frame format. These transponders are quite expensive and their
lack of transparency also constrains the rapid introduction of new
services. Thus there are strong motivators to introduce "domains of
transparency" - all-optical subnetworks - larger than an OTS.
The routing of lightpaths through an all-optical network has received
extensive attention. (See [Yates99] or [Ramaswami98]). When
discussing routing in an all-optical network it is usually assumed
that all routes have adequate signal quality. This may be ensured by
limiting all-optical networks to subnetworks of limited geographic
size that are optically isolated from other parts of the optical
layer by transponders. This approach is very practical and has been
applied to date, e.g., when determining the maximum length of an
Optical Transport System (OTS). Furthermore operational
considerations like fault isolation also make limiting the size of
domains of transparency attractive.
There are however reasons to consider contained domains of
transparency in which not all routes have adequate signal quality.
From a demand perspective, maximum bit rates have rapidly increased
from DS3 to OC-192 and soon OC-768 (40 Gb/sec). As bit rates
increase it is necessary to increase power. This makes impairments
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and nonlinearities more troublesome. From a supply perspective,
optical technology is advancing very rapidly, making ever-larger
domains possible. In this section, we assume that these
considerations will lead to the deployment of a domain of
transparency that is too large to ensure that all potential routes
have adequate signal quality for all circuits. Our goal is to
understand the impacts of the various types of impairments in this
environment.
Note that, as we describe later in the section, there are many types
of physical impairments. Which of these needs to be dealt with
explicitly when performing on-line distributed routing will vary
considerably and will depend on many variables, including:
- Equipment vendor design choices,
- Fiber characteristics,
- Service characteristics (e.g., circuit speeds),
- Network size,
- Network operator engineering and deployment strategies.
For example, a metropolitan network that does not intend to support
bit rates above 2.5 Gb/sec may not be constrained by any of these
impairments, while a continental or international network that wished
to minimize O/E/O regeneration investment and support 40 Gb/sec
connections might have to explicitly consider many of them. Also, a
network operator may reduce or even eliminate their constraint set by
building a relatively small domain of transparency to ensure that all
the paths are feasible, or by using some proprietary tools based on
rules from the OTS vendor to pre-qualify paths between node pairs and
put them in a table that can be accessed each time a routing decision
has to be made through that domain.
4.1. Problem Formulation
We consider a single domain of transparency without wavelength
translation. Additionally, due to the proprietary nature of DWDM
transmission technology, we assume that the domain is either single
vendor or architected using a single coherent design, particularly
with regard to the management of impairments.
We wish to route a unidirectional circuit from ingress client node X
to egress client node Y. At both X and Y, the circuit goes through
an O/E/O conversion that optically isolates the portion within our
domain. We assume that we know the bit rate of the circuit. Also,
we assume that the adaptation function at X may apply some Forward
Error Correction (FEC) method to the circuit. We also assume we know
the launch power of the laser at X.
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Impairments can be classified into two categories, linear and
nonlinear. (See [Tkach98] or [Kaminow02] for more on impairment
constraints.) Linear effects are independent of signal power and
affect wavelengths individually. Amplifier spontaneous emission
(ASE), polarization mode dispersion (PMD), and chromatic dispersion
are examples. Nonlinearities are significantly more complex: they
generate not only impairments on each channel, but also crosstalk
between channels.
In the remainder of this section we first outline how two key linear
impairments (PMD and ASE) might be handled by a set of analytical
formulae as additional constraints on routing. We next discuss how
the remaining constraints might be approached. Finally we take a
broader perspective and discuss the implications of such constraints
on control plane architecture and also on broader constrained domain
of transparency architecture issues.
4.2. Polarization Mode Dispersion (PMD)
For a transparent fiber segment, the general PMD requirement is that
the time-average differential group delay (DGD) between two
orthogonal state of polarizations should be less than some fraction a
of the bit duration, T=1/B, where B is the bit rate. The value of
the parameter a depends on three major factors: 1) margin allocated
to PMD, e.g., 1dB; 2) targeted outage probability, e.g., 4x10-5, and
3) sensitivity of the receiver to DGD. A typical value for a is 10%
[ITU]. More aggressive designs to compensate for PMD may allow
values higher than 10%. (This would be a system parameter dependent
on the system design. It would need to be known to the routing
process.)
The PMD parameter (Dpmd) is measured in pico-seconds (ps) per
sqrt(km). The square of the PMD in a fiber span, denoted as span-
PMD-square is then given by the product of Dpmd**2 and the span
length. (A fiber span in a transparent network refers to a segment
between two optical amplifiers.) If Dpmd is constant, this results
in a upper bound on the maximum length of an M-fiber-span transparent
segment, which is inversely proportional to the square of the product
of bit rate and Dpmd (the detailed equation is omitted due to the
format constraint - see [Strand01] for details).
For older fibers with a typical PMD parameter of 0.5 picoseconds per
square root of km, based on the constraint, the maximum length of the
transparent segment should not exceed 400km and 25km for bit rates of
10Gb/s and 40Gb/s, respectively. Due to recent advances in fiber
technology, the PMD-limited distance has increased dramatically. For
newer fibers with a PMD parameter of 0.1 picosecond per square root
of km, the maximum length of the transparent segment (without PMD
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compensation) is limited to 10000km and 625km for bit rates of 10Gb/s
and 40Gb/, respectively. Still lower values of PMD are attainable in
commercially available fiber today, and the PMD limit can be further
extended if a larger value of the parameter a (ratio of DGD to the
bit period) can be tolerated. In general, the PMD requirement is not
an issue for most types of fibers at 10Gb/s or lower bit rate. But
it will become an issue at bit rates of 40Gb/s and higher.
If the PMD parameter varies between spans, a slightly more
complicated equation results (see [Strand01]), but in any event the
only link dependent information needed by the routing algorithm is
the square of the link PMD, denoted as link-PMD-square. It is the
sum of the span-PMD-square of all spans on the link.
Note that when one has some viable PMD compensation devices and
deploy them ubiquitously on all routes with potential PMD issues in
the network, then the PMD constraint disappears from the routing
perspective.
4.3. Amplifier Spontaneous Emission
ASE degrades the optical signal to noise ratio (OSNR). An acceptable
optical SNR level (SNRmin), which depends on the bit rate,
transmitter-receiver technology (e.g., FEC), and margins allocated
for the impairments, needs to be maintained at the receiver. In
order to satisfy this requirement, vendors often provide some general
engineering rule in terms of maximum length of the transparent
segment and number of spans. For example, current transmission
systems are often limited to up to 6 spans each 80km long. For
larger transparent domains, more detailed OSNR computations will be
needed to determine whether the OSNR level through a domain of
transparency is acceptable. This would provide flexibility in
provisioning or restoring a lightpath through a transparent
subnetwork.
Assume that the average optical power launched at the transmitter is
P. The lightpath from the transmitter to the receiver goes through M
optical amplifiers, with each introducing some noise power. Unity
gain can be used at all amplifier sites to maintain constant signal
power at the input of each span to minimize noise power and
nonlinearity. A constraint on the maximum number of spans can be
obtained [Kaminow97] which is proportional to P and inversely
proportional to SNRmin, optical bandwidth B, amplifier gain G-1 and
spontaneous emission factor n of the optical amplifier, assuming all
spans have identical gain and noise figure. (Again, the detailed
equation is omitted due to the format constraint - see [Strand01] for
details.) Let's take a typical example. Assuming P=4dBm,
SNRmin=20dB with FEC, B=12.5GHz, n=2.5, G=25dB, based on the
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constraint, the maximum number of spans is at most 10. However, if
FEC is not used and the requirement on SNRmin becomes 25dB, the
maximum number of spans drops down to 3.
For ASE the only link-dependent information needed by the routing
algorithm is the noise of the link, denoted as link-noise, which is
the sum of the noise of all spans on the link. Hence the constraint
on ASE becomes that the aggregate noise of the transparent segment
which is the sum of the link-noise of all links can not exceed
P/SNRmin.
4.4. Approximating the Effects of Some Other Impairment Constraints
There are a number of other impairment constraints that we believe
could be approximated with a domain-wide margin on the OSNR, plus in
some cases a constraint on the total number of networking elements
(OXC or OADM) along the path. Most impairments generated at OXCs or
OADMs, including polarization dependent loss, coherent crosstalk, and
effective passband width, could be dealt with using this approach.
In principle, impairments generated at the nodes can be bounded by
system engineering rules because the node elements can be designed
and specified in a uniform manner. This approach is not feasible
with PMD and noise because neither can be uniformly specified.
Instead, they depend on node spacing and the characteristics of the
installed fiber plant, neither of which are likely to be under the
system designer's control.
Examples of the constraints we propose to approximate with a domain-
wide margin are given in the remaining paragraphs in this section.
It should be kept in mind that as optical transport technology
evolves it may become necessary to include some of these impairments
explicitly in the routing process. Other impairments not mentioned
here at all may also become sufficiently important to require
incorporation either explicitly or via a domain-wide margin.
Other Polarization Dependent Impairments
Other polarization-dependent effects besides PMD influence system
performance. For example, many components have polarization-
dependent loss (PDL) [Ramaswami98], which accumulates in a system
with many components on the transmission path. The state of
polarization fluctuates with time and its distribution is very
important also. It is generally required that the total PDL on
the path be maintained within some acceptable limit, potentially
by using some compensation technology for relatively long
transmission systems, plus a small built-in margin in OSNR. Since
the total PDL increases with the number of components in the data
path, it must be taken into account by the system vendor when
determining the maximum allowable number of spans.
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Chromatic Dispersion
In general this impairment can be adequately (but not optimally)
compensated for on a per-link basis, and/or at system initial
setup time. Today most deployed compensation devices are based on
Dispersion Compensation Fiber (DCF). DCF provides per fiber
compensation by means of a spool of fiber with a CD coefficient
opposite to the fiber. Due to the imperfect matching between the
CD slope of the fiber and the DCF some lambdas can be over
compensated while others can be under compensated. Moreover DCF
modules may only be available in fixed lengths of compensating
fiber; this means that sometimes it is impossible to find a DCF
module that exactly compensates the CD introduced by the fiber.
These effects introduce what is known as residual CD. Residual CD
varies with the frequency of the wavelength. Knowing the
characteristics of both of the fiber and the DCF modules along the
path, this can be calculated with a sufficient degree of
precision. However this is a very challenging task. In fact the
per-wavelength residual dispersion needs to be combined with other
information in the system (e.g., types fibers to figure out the
amount of nonlinearities) to obtain the net effect of CD either by
simulation or by some analytical approximation. It appears that
the routing/control plane should not be burdened by such a large
set of information while it can be handled at the system design
level. Therefore it will be assumed until proven otherwise that
residual dispersion should not be reported. For high bit rates,
dynamic dispersion compensation may be required at the receiver to
clean up any residual dispersion.
Crosstalk
Optical crosstalk refers to the effect of other signals on the
desired signal. It includes both coherent (i.e., intrachannel)
crosstalk and incoherent (i.e., interchannel) crosstalk. Main
contributors of crosstalk are the OADM and OXC sites that use a
DWDM multiplexer/demultiplexer (MUX/DEMUX) pair. For a relatively
sparse network where the number of OADM/OXC nodes on a path is
low, crosstalk can be treated with a low margin in OSNR without
being a binding constraint. But for some relatively dense
networks where crosstalk might become a binding constraint, one
needs to propagate the per-link crosstalk information to make sure
that the end-to-end path crosstalk which is the sum of the
crosstalks on all the corresponding links to be within some limit,
e.g., -25dB threshold with 1dB penalty ([Goldstein94]). Another
way to treat it without having to propagate per-link crosstalk
information is to have the system evaluate what the maximum number
of OADM/OXC nodes that has a MUX/DEMUX pair for the worst route in
the transparent domain for a low built-in margin. The latter one
should work well where all the OXC/OADM nodes have similar level
of crosstalk.
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Effective Passband
As more and more DWDM components are cascaded, the effective
passband narrows. The number of filters along the link, their
passband width and their shape will determine the end-to-end
effective passband. In general, this is a system design issue,
i.e., the system is designed with certain maximum bit rate using
the proper modulation format and filter spacing. For linear
systems, the filter effect can be turned into a constraint on the
maximum number of narrow filters with the condition that filters
in the systems are at least as wide as the one in the receiver.
Because traffic at lower bit rates can tolerate a narrower
passband, the maximum allowable number of narrow filters will
increase as the bit rate decreases.
Nonlinear Impairments
It seems unlikely that these can be dealt with explicitly in a
routing algorithm because they lead to constraints that can couple
routes together and lead to complex dependencies, e.g., on the
order in which specific fiber types are traversed [Kaminow97].
Note that different fiber types (standard single mode fiber,
dispersion shifted fiber, dispersion compensated fiber, etc.) have
very different effects from nonlinear impairments. A full
treatment of the nonlinear constraints would likely require very
detailed knowledge of the physical infrastructure, including
measured dispersion values for each span, fiber core area and
composition, as well as knowledge of subsystem details such as
dispersion compensation technology. This information would need
to be combined with knowledge of the current loading of optical
signals on the links of interest to determine the level of
nonlinear impairment. Alternatively, one could assume that
nonlinear impairments are bounded and result in X dB margin in the
required OSNR level for a given bit rate, where X for performance
reasons would be limited to 1 or 2 dB, consequently setting a
limit on the maximum number of spans. For the approach described
here to be useful, it is desirable for this span length limit to
be longer than that imposed by the constraints which can be
treated explicitly. When designing a DWDM transport system, there
are tradeoffs between signal power launched at the transmitter,
span length, and nonlinear effects on BER that need to be
considered jointly. Here, we assume that an X dB margin is
obtained after the transport system has been designed with a fixed
signal power and maximum span length for a given bit rate. Note
that OTSs can be designed in very different ways, in linear,
pseudo-linear, or nonlinear environments. The X-dB margin
approach may be valid for some but not for others. However, it is
likely that there is an advantage in designing systems that are
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less aggressive with respect to nonlinearities, and therefore
somewhat sub-optimal, in exchange for improved scalability,
simplicity and flexibility in routing and control plane design.
4.5. Other Impairment Considerations
There are many other types of impairments that can degrade
performance. In this section, we briefly mention one other type of
impairment, which we propose be dealt with by either the system
designer or by the transmission engineers at the time the system is
installed. If dealt with successfully in this manner they should not
need to be considered in the dynamic routing process.
Gain Nonuniformity and Gain Transients For simple noise estimates to
be of use, the amplifiers must be gain-flattened and must have
automatic gain control (AGC). Furthermore, each link should have
dynamic gain equalization (DGE) to optimize power levels each time
wavelengths are added or dropped. Variable optical attenuators on
the output ports of an OXC or OADM can be used for this purpose, and
in-line devices are starting to become commercially available.
Optical channel monitors are also required to provide feedback to the
DGEs. AGC must be done rapidly if signal degradation after a
protection switch or link failure is to be avoided.
Note that the impairments considered here are treated more or less
independently. By considering them jointly and varying the tradeoffs
between the effects from different components may allow more routes
to be feasible. If that is desirable or the system is designed such
that certain impairments (e.g., nonlinearities) need to be considered
by a centralized process, then distributed routing is not the one to
use.
4.6. An Alternative Approach - Using Maximum Distance as the Only Constraint
Today, carriers often use maximum distance to engineer point-to-point
OTS systems given a fixed per-span length based on the OSNR
constraint for a given bit rate. They may desire to keep the same
engineering rule when they move to all-optical networks. Here, we
discuss the assumptions that need to be satisfied to keep this
approach viable and how to treat the network elements between two
adjacent links.
In order to use the maximum distance for a given bit rate to meet an
OSNR constraint as the only binding constraint, the operators need to
satisfy the following constraints in their all-optical networks:
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- All the other non-OSNR constraints described in the previous
subsections are not binding factors as long as the maximum
distance constraint is met.
- Specifically for PMD, this means that the whole all-optical
network is built on top of sufficiently low-PMD fiber such that
the upper bound on the mean aggregate path DGD is always satisfied
for any path that does not exceed the maximum distance, or PMD
compensation devices might be used for routes with high-PMD
fibers.
- In terms of the ASE/OSNR constraint, in order to convert the ASE
constraint into a distance constraint directly, the network needs
to have a fixed fiber distance D for each span (so that ASE can be
directly mapped by the gain of the amplifier which equals to the
loss of the previous fiber span), e.g., 80km spacing which is
commonly chosen by carriers. However, when spans have variable
lengths, certain adjustment and compromise need to be made in
order to avoid treating ASE explicitly as in section 4.3. These
include: 1) Unless a certain mechanism is built in the OTS to take
advantage of shorter spans, spans shorter than a typical span
length D need to be treated as a span of length D instead of with
its real length. 2) Spans that are longer than D would have a
higher average span loss. In general, the maximum system reach
decreases when the average span loss increases. Thus, in order to
accommodate longer spans in the network, the maximum distance
upper bound has to be set with respect to the average span loss of
the worst path in the network. This sub-optimality may be
acceptable for some networks if the variance is not too large, but
may be too conservative for others.
If these assumptions are satisfied, the second issue we need to
address is how to treat a transparent network element (e.g., MEMS-
based switch) between two adjacent links in terms of a distance
constraint since it also introduces an insertion loss. If the
network element cannot somehow compensate for this OSNR degradation,
one approach is to convert each network element into an equivalent
length of fiber based on its loss/ASE contribution. Hence, in
general, introducing a set of transparent network elements would
effectively result in reducing the overall actual transmission
distance between the OEO edges.
With this approach, the link-specific state information is link-
distance, the length of a link. It equals the distance sum of all
fiber spans on the link and the equivalent length of fiber for the
network element(s) on the link. The constraint is that the sum of
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all the link-distance over all links of a path should be less than
the maximum-path-distance, the upper bound of all paths.
4.7. Other Considerations
Routing in an all-optical network without wavelength conversion
raises several additional issues:
- Since the route selected must have the chosen wavelength available
on all links, this information needs to be considered in the
routing process. One approach is to propagate information
throughout the network about the state of every wavelength on
every link in the network. However, the state required and the
overhead involved in processing and maintaining this information
is proportional to the total number of links (thus, number of
nodes squared), maximum number of wavelengths (which keeps
doubling every couple of years), and the frequency of wavelength
availability changes, which can be very high. Instead
[Hjalmtysson00], proposes an alternative method which probes along
a chosen path to determine which wavelengths (if any) are
available. This would require a significant addition to the
routing logic normally used in OSPF. Others have proposed
simultaneously probing along multiple paths.
- Choosing a path first and then a wavelength along the path is
known to give adequate results in simple topologies such as rings
and trees ([Yates99]). This does not appear to be true in large
mesh networks under realistic provisioning scenarios, however.
Instead significantly better results are achieved if wavelength
and route are chosen simultaneously ([Strand01b]). This approach
would however also have a significant effect on OSPF.
4.8. Implications For Routing and Control Plane Design
If distributed routing is desired, additional state information will
be required by the routing to deal with the impairments described in
Sections 4.2 - 4.4:
- As mentioned earlier, an operator who wants to avoid having to
provide impairment-related parameters to the control plane may
elect not to deal with them at the routing level, instead treating
them at the system design and planning level if that is a viable
approach for their network. In this approach the operator can
pre-qualify all or a set of feasible end-to-end optical paths
through the domain of transparency for each bit rate. This
approach may work well with relatively small and sparse networks,
but it may not be scalable for large and dense networks where the
number of feasible paths can be very large.
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- If the optical paths are not pre-qualified, additional link-
specific state information will be required by the routing
algorithm for each type of impairment that has the potential of
being limiting for some routes. Note that for one operator, PMD
might be the only limiting constraint while for another, ASE might
be the only one, or it could be both plus some other constraints
considered in this document. Some networks might not be limited
by any of these constraints.
- For an operator needing to deal explicitly with these constraints,
the link-dependent information identified above for PMD is link-
PMD-square which is the square of the total PMD on a link. For
ASE the link-dependent information identified is link-noise which
is the total noise on a link. Other link-dependent information
includes link-span-length which is the total number of spans on a
link, link-crosstalk or OADM-OXC-number which is the total
crosstalk or the number of OADM/OXC nodes on a link, respectively,
and filter-number which is the number of narrow filters on a link.
When the alternative distance-only approach is chosen, the link-
specific information is link-distance.
- In addition to the link-specific information, bounds on each of
the impairments need to be quantified. Since these bounds are
determined by the system designer's impairment allocations, these
will be system dependent. For PMD, the constraint is that the sum
of the link-PMD-square of all links on the transparent segment is
less than the square of (a/B) where B is the bit rate. Hence, the
required information is the parameter "a". For ASE, the
constraint is that the sum of the link-noise of all links is no
larger than P/SNRmin. Thus, the information needed include the
launch power P and OSNR requirement SNRmin. The minimum
acceptable OSNR, in turn, depends on the strength of the FEC being
used and the margins reserved for other types of impairments.
Other bounds include the maximum span length of the transmission
system, the maximum path crosstalk or the maximum number of
OADM/OXC nodes, and the maximum number of narrow filters, all are
bit rate dependent. With the alternative distance-only approach,
the upper bound is the maximum-path-distance. In single-vendor
"islands" some of these parameters may be available in a local or
EMS database and would not need to be advertised
- It is likely that the physical layer parameters do not change
value rapidly and could be stored in some database; however these
are physical layer parameters that today are frequently not known
at the granularity required. If the ingress node of a lightpath
does path selection these parameters would need to be available at
this node.
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- The specific constraints required in a given situation will depend
on the design and engineering of the domain of transparency; for
example it will be essential to know whether chromatic dispersion
has been dealt with on a per-link basis, and whether the domain is
operating in a linear or nonlinear regime.
- As optical transport technology evolves, the set of constraints
that will need to be considered either explicitly or via a
domain-wide margin may change. The routing and control plane
design should therefore be as open as possible, allowing
parameters to be included as necessary.
- In the absence of wavelength conversion, the necessity of finding
a single wavelength that is available on all links introduces the
need to either advertise detailed information on wavelength
availability, which probably doesn't scale, or have some mechanism
for probing potential routes with or without crankback to
determine wavelength availability. Choosing the route first, and
then the wavelength, may not yield acceptable utilization levels
in mesh-type networks.
5. More Complex Networks
Mixing optical equipment in a single domain of transparency that has
not been explicitly designed to interwork is beyond the scope of this
document. This includes most multi-vendor all-optical networks.
An optical network composed of multiple domains of transparency
optically isolated from each other by O/E/O devices (transponders) is
more plausible. A network composed of both "opaque" (optically
isolated) OLXCs and one or more all-optical "islands" isolated by
transponders is of particular interest because this is most likely
how all-optical technologies (such as that described in Sec. 2) are
going to be introduced. (We use the term "island" in this discussion
rather than a term like "domain" or "area" because these terms are
associated with specific approaches like BGP or OSPF.)
We consider the complexities raised by these alternatives now.
The first requirement for routing in a multi-island network is that
the routing process needs to know the extent of each island. There
are several reasons for this:
- When entering or leaving an all-optical island, the regeneration
process cleans up the optical impairments discussed in Sec. 3.
- Each all-optical island may have its own bounds on each
impairment.
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- The routing process needs to be sensitive to the costs associated
with "island-hopping".
This last point needs elaboration. It is extremely important to
realize that, at least in the short to intermediate term, the
resources committed by a single routing decision can be very
significant: The equipment tied up by a single coast-to-coast OC-192
can easily have a first cost of $10**6, and the holding times on a
circuit once established is likely to be measured in months.
Carriers will expect the routing algorithms used to be sensitive to
these costs. Simplistic measures of cost such as the number of
"hops" are not likely to be acceptable.
Taking the case of an all-optical island consisting of an "ultra
long-haul" system like that in Fig. 3-1 embedded in an OEO network of
electrical fabric OLXCs as an example: It is likely that the ULH
system will be relatively expensive for short hops but relatively
economical for longer distances. It is therefore likely to be
deployed as a sort of "express backbone". In this scenario a carrier
is likely to expect the routing algorithm to balance OEO costs
against the additional costs associated with ULH technology and route
circuitously to make maximum use of the backbone where appropriate.
Note that the metrics used to do this must be consistent throughout
the routing domain if this expectation is to be met.
The first-order implications for GMPLS seem to be:
- Information about island boundaries needs to be advertised.
- The routing algorithm needs to be sensitive to island transitions
and to the connectivity limitations and impairment constraints
particular to each island.
- The cost function used in routing must allow the balancing of
transponder costs, OXC and OADM costs, and line haul costs across
the entire routing domain.
Several distributed approaches to multi-island routing seem worth
investigating:
- Advertise the internal topology and constraints of each island
globally; let the ingress node compute an end-to-end strict
explicit route sensitive to all constraints and wavelength
availabilities. In this approach the routing algorithm used by
the ingress node must be able to deal with the details of routing
within each island.
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- Have the EMS or control plane of each island determine and
advertise the connectivity between its boundary nodes together
with additional information such as costs and the bit rates and
formats supported. As the spare capacity situation changes,
updates would be advertised. In this approach impairment
constraints are handled within each island and impairment-related
parameters need not be advertised outside of the island. The
ingress node would then do a loose explicit route and leave the
routing and wavelength selection within each island to the island.
- Have the ingress node send out probes or queries to nearby gateway
nodes or to an NMS to get routing guidance.
6. Diversity6.1. Background on Diversity
"Diversity" is a relationship between lightpaths. Two lightpaths are
said to be diverse if they have no single point of failure. In
traditional telephony the dominant transport failure mode is a
failure in the interoffice plant, such as a fiber cut inflicted by a
backhoe.
Why is diversity a unique problem that needs to be considered for
optical networks? Traditionally, data network operators have relied
on their private line providers to ensure diversity and so have not
had to deal directly with the problem. GMPLS makes the complexities
handled by the private line provisioning process, including
diversity, part of the common control plane and so visible to all.
To determine whether two lightpath routings are diverse it is
necessary to identify single points of failure in the interoffice
plant. To do so we will use the following terms: A fiber cable is a
uniform group of fibers contained in a sheath. An Optical Transport
System will occupy fibers in a sequence of fiber cables. Each fiber
cable will be placed in a sequence of conduits - buried honeycomb
structures through which fiber cables may be pulled - or buried in a
right of way (ROW). A ROW is land in which the network operator has
the right to install his conduit or fiber cable. It is worth noting
that for economic reasons, ROWs are frequently obtained from
railroads, pipeline companies, or thruways. It is frequently the
case that several carriers may lease ROW from the same source; this
makes it common to have a number of carriers' fiber cables in close
proximity to each other. Similarly, in a metropolitan network,
several carriers might be leasing duct space in the same RBOC
conduit. There are also "carrier's carriers" - optical networks
which provide fibers to multiple carriers, all of whom could be
affected by a single failure in the "carrier's carrier" network. In
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a typical intercity facility network there might be on the order of
100 offices that are candidates for OLXCs. To represent the inter-
office fiber network accurately a network with an order of magnitude
more nodes is required. In addition to Optical Amplifier (OA) sites,
these additional nodes include:
- Places where fiber cables enter/leave a conduit or right of way;
- Locations where fiber cables cross; Locations where fiber splices
are used to interchange fibers between fiber cables.
An example of the first might be:
A B
A-------------B \ /
\ /
X-----Y
/ \
C-------------D / \
C D
(a) Fiber Cable Topology (b) Right-Of-Way/Conduit Topology
Figure 6-1: Fiber Cable vs. ROW Topologies
Here the A-B fiber cable would be physically routed A-X-Y-B and the
C-D cable would be physically routed C-X-Y-D. This topology might
arise because of some physical bottleneck: X-Y might be the Lincoln
Tunnel, for example, or the Bay Bridge.
Fiber route crossing (the second case) is really a special case of
this, where X and Y coincide. In this case the crossing point may
not even be a manhole; the fiber routes might just be buried at
different depths.
Fiber splicing (the third case) often occurs when a major fiber route
passes near to a small office. To avoid the expense and additional
transmission loss only a small number of fibers are spliced out of
the major route into a smaller route going to the small office. This
might well occur in a manhole or hut. An example is shown in Fig.
6-2(a), where A-X-B is the major route, X the manhole, and C the
smaller office. The actual fiber topology would then look like Fig.
6-2(b), where there would typically be many more A-B fibers than A-C
or C-B fibers, and where A-C and C-B might have different numbers of
fibers. (One of the latter might even be missing.)
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C C
| / \
| / \
| / \
A------X------B A---------------B
(a) Fiber Cable Topology (b) Fiber Topology
Figure 6-2. Fiber Cable vs Fiber Topologies
The imminent deployment of ultra-long (>1000 km) Optical Transport
Systems introduces a further complexity: Two OTSes could interact a
number of times. To make up a hypothetical example: A New York -
Atlanta OTS and a Philadelphia - Orlando OTS might ride on the same
right of way for x miles in Maryland and then again for y miles in
Georgia. They might also cross at Raleigh or some other intermediate
node without sharing right of way.
Diversity is often equated to routing two lightpaths between a single
pair of points, or different pairs of points so that no single route
failure will disrupt them both. This is too simplistic, for a number
of reasons:
- A sophisticated client of an optical network will want to derive
diversity needs from his/her end customers' availability
requirements. These often lead to more complex diversity
requirements than simply providing diversity between two
lightpaths. For example, a common requirement is that no single
failure should isolate a node or nodes. If a node A has single
lightpaths to nodes B and C, this requires A-B and A-C to be
diverse. In real applications, a large data network with N
lightpaths between its routers might describe their needs in an
NxN matrix, where (i,j) defines whether lightpaths i and j must be
diverse.
- Two circuits that might be considered diverse for one application
might not be considered diverse for in another situation.
Diversity is usually thought of as a reaction to interoffice route
failures. High reliability applications may require other types
of failures to be taken into account. Some examples:
o Office Outages: Although less frequent than route failures,
fires, power outages, and floods do occur. Many network
managers require that diverse routes have no (intermediate)
nodes in common. In other cases an intermediate node might be
acceptable as long as there is power diversity within the
office.
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o Shared Rings: Many applications are willing to allow "diverse"
circuits to share a SONET ring-protected link; presumably they
would allow the same for optical layer rings.
o Disasters: Earthquakes and floods can cause failures over an
extended area. Defense Department circuits might need to be
routed with nuclear damage radii taken into account.
- Conversely, some networks may be willing to take somewhat larger
risks. Taking route failures as an example: Such a network might
be willing to consider two fiber cables in heavy duty concrete
conduit as having a low enough chance of simultaneous failure to
be considered "diverse". They might also be willing to view two
fiber cables buried on opposite sides of a railroad track as being
diverse because there is minimal danger of a single backhoe
disrupting them both even though a bad train wreck might
jeopardize them both. A network seeking N mutually diverse paths
from an office with less than N diverse ROWs will need to live
with some level of compromise in the immediate vicinity of the
office.
These considerations strongly suggest that the routing algorithm
should be sensitive to the types of threat considered unacceptable by
the requester. Note that the impairment constraints described in the
previous section may eliminate some of the long circuitous routes
sometimes needed to provide diversity. This would make it harder to
find many diverse paths through an all-optical network than an opaque
one.
[Hjalmtysson00] introduced the term "Shared Risk Link Group" (SRLG)
to describe the relationship between two non-diverse links. The
above examples and discussion given at the start of this section
suggests that an SRLG should be characterized by 2 parameters:
- Type of Compromise: Examples would be shared fiber cable, shared
conduit, shared ROW, shared optical ring, shared office without
power sharing, etc.)
- Extent of Compromise: For compromised outside plant, this would
be the length of the sharing.
A CSPF algorithm could then penalize a diversity compromise by an
amount dependent on these two parameters.
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RFC 4054 Optical Layer Routing May 2005
Two links could be related by many SRLGs. (AT&T's experience
indicates that a link may belong to over 100 SRLGs, each
corresponding to a separate fiber group.) Each SRLG might relate a
single link to many other links. For the optical layer, similar
situations can be expected where a link is an ultra-long OTS.
The mapping between links and different types of SRLGs is in general
defined by network operators based on the definition of each SRLG
type. Since SRLG information is not yet ready to be discoverable by
a network element and does not change dynamically, it need not be
advertised with other resource availability information by network
elements. It could be configured in some central database and be
distributed to or retrieved by the nodes, or advertised by network
elements at the topology discovery stage.
6.2. Implications For Routing
Dealing with diversity is an unavoidable requirement for routing in
the optical layer. It requires dealing with constraints in the
routing process, but most importantly requires additional state
information (e.g., the SRLG relationships). The routings of any
existing circuits from which the new circuit must be diverse must
also be available to the routing process.
At present SRLG information cannot be self-discovered. Indeed, in a
large network it is very difficult to maintain accurate SRLG
information. The problem becomes particularly daunting whenever
multiple administrative domains are involved, for instance after the
acquisition of one network by another, because there normally is a
likelihood that there are diversity violations between the domains.
It is very unlikely that diversity relationships between carriers
will be known any time in the near future.
Considerable variation in what different customers will mean by
acceptable diversity should be anticipated. Consequently we suggest
that an SRLG should be defined as follows: (i) It is a relationship
between two or more links, and (ii) it is characterized by two
parameters, the type of compromise (shared conduit, shared ROW,
shared optical ring, etc.) and the extent of the compromise (e.g.,
the number of miles over which the compromise persisted). This will
allow the SRLGs appropriate to a particular routing request to be
easily identified.
7. Security Considerations
We are assuming OEO interfaces to the domain(s) covered by our
discussion (see, e.g., Sec. 4.1 above). If this assumption were to
be relaxed and externally generated optical signals allowed into the
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domain, network security issues would arise. Specifically,
unauthorized usage in the form of signals at improper wavelengths or
with power levels or impairments inconsistent with those assumed by
the domain would be possible. With OEO interfaces, these types of
layer one threats should be controllable.
A key layer one security issue is resilience in the face of physical
attack. Diversity, as describe in Sec. 6, is a part of the solution.
However, it is ineffective if there is not sufficient spare capacity
available to make the network whole after an attack. Several major
related issues are:
- Defining the threat: If, for example, an electro-magnetic
interference (EMI) burst is an in-scope threat, then (in the
terminology of Sec. 6) all of the links sufficiently close
together to be disrupted by such a burst must be included in a
single SRLG. Similarly for other threats: For each in-scope
threat, SRLGs must be defined so that all links vulnerable to a
single incident of the threat must be grouped together in a single
SRLG.
- Allocating responsibility for responding to a layer one failure
between the various layers (especially the optical and IP layers):
This must be clearly specified to avoid churning and unnecessary
service interruptions.
The whole proposed process depends on the integrity of the impairment
characterization information (PMD parameters, etc.) and also the SRLG
definitions. Security of this information, both when stored and when
distributed, is essential.
This document does not address control plane issues, and so control-
plane security is out of scope. IPO control plane security
considerations are discussed in [Rajagopalam04]. Security
considerations for GMPLS, a likely control plane candidate, are
discussed in [Mannie04].
8. Acknowledgments
This document has benefited from discussions with Michael Eiselt,
Jonathan Lang, Mark Shtaif, Jennifer Yates, Dongmei Wang, Guangzhi
Li, Robert Doverspike, Albert Greenberg, Jim Maloney, John Jacob,
Katie Hall, Diego Caviglia, D. Papadimitriou, O. Audouin, J. P.
Faure, L. Noirie, and with our OIF colleagues.
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